1. Field of the Invention
The present invention relates to doped silicon films formed in integrated circuit semiconductors, and in particular, the invention relates to doped silicon films formed in trench structures, a preferred application being a doped silicon film formed in a deep trench structure to achieve improved trench capacitors for DRAMs.
2. Description of Related Art
Deep and shallow trenches are used in the process of manufacturing dynamic random access memories (DRAMs). Deep trenches are preferably used in forming trench type capacitors, and shallow trenches are preferably used in forming electrical isolation barriers between semiconductor regions. Shallow trench isolation techniques (sometimes referred to as STI techniques) include filling the trench with an electrical insulator, for example, silicon dioxide. In contrast, deep trench capacitor structures are formed with the trench filled with electrically conductive materials, for example, doped poly-silicon. The capacitor structure includes a conductive layer to function as a capacitor plate, an insulator layer to function as a capacitor dielectric and another conductive layer to function as a capacitor electrode or storage node. Deep trenches are preferred over shallow trenches to form capacitors since the deeper the trench the greater is the capacitive area between the capacitor plate and the capacitor electrode.
In U.S. Pat. No. 4,971,926, granted Nov. 20, 1990 to Kinugawa, a method of manufacturing a semiconductor device is described as using deep trench technology for both isolation barriers and trench capacitors. FIG. 7 of the present application corresponds to FIG. 2 of the Kinugawa patent. In FIG. 7, a DRAM memory cell capacitor element illustrates an application of the groove or deep trench technology. A field oxide film 12 is formed by a selective oxidation process on the surface of silicon substrate 11. The field oxide film 12 isolates a surrounded memory cell region. A groove having a substantially rectangular sectional profile is formed by a reactive ion etching process in the memory cell region. An n type impurity region 15 is formed by doping the silicon substrate 11 with an n type impurity from the wall surface of the groove. An oxide film 13 is formed by thermal oxidation. An electrode 14 consisting of a polycrystalline silicon layer, doped to be conductive, is formed over the thermal oxide film 13 in the groove. Transistor 16 is a transfer transistor of the DRAM memory cell. By forming the capacitor element so as to use the wall surface of the groove, it is possible to reduce the element area exclusively occupied by the capacitor element and hence increase the integration density. This method of improving the integration density by making use of a groove having walls perpendicular to the principal surface of the substrate also finds extensive applications to resistor elements.
Formation of capacitor electrode 14 is a critical process. It is generally difficult but desirable to form conformal films as electrode 14 when using an insitu doped polycrystalline silicon technique. To obtain a conformal film for use as capacitor electrodes, the present inventors have used a layer process. The layer process includes steps of first depositing intrinsic amorphus silicon layer, then depositing a dopant (e.g., arsenic or phosphorus) layer, and finally depositing another intrinsic amorphus silicon layer. This results in a conformal process which capable of filling deep trenches.
FIGS. 4A-4C and FIGS. 5A-5D illustrate this layer process for forming doped silicon films in deep trenches. In FIG. 4A, dielectric 102 (e.g., silicon dioxide or silicon nitride) is formed on silicon substrate 100. Thereafter, amorphus silicon 104 is formed on dielectric 102. In FIG. 4B, dopant film 106 is formed on amorphus silicon film 104. In FIG. 4C, amorphus silicon film 112 is formed on dopant film 106. Subsequent heating of such a structure diffuses the dopant into the amorphus so as to render conductive the films 104, 106 and 112. However, in FIG. 4C, rough surface 114 results from this method of forming doped silicon films.
This method of film formation may be applied to formation of a trench capacitor structure. In FIG. 5A, doped silicon substrate 100, preferably n type semiconductor, is processed to form thereon silicon dioxide pad layer 101A, silicon nitride pad layer 101B and silicon dioxide mask layer 101C. A deep trench is etched, preferably by reactive ion etching (RIE), using silicon dioxide mask 101C to define the trench pattern. Typically, the trench is etched to a depth of from 4 to 8 microns, but deeper or shallower trenches may be used.
Outdiffusing a dopant (e.g., arsenic) in substrate 100 may be used to form capacitor plate 98 (e.g., of n.sup.+ type semiconductor). Dielectric 102 (e.g., silicon dioxide or silicon nitride as in FIG. 4A) is conformally formed in the trench on silicon substrate 100 as a storage node dielectric as shown in FIG. 5B.
The process for forming a doped silicon film used by the present inventors to obtain a conformal film for use as a capacitor electrode is applied on storage node electrode dielectric 102 (FIGS. 4A and 5B). First, amorphus silicon layer 104 is formed on dielectric 102 (FIG. 5B). Then, dopant film 106 (e.g., arsenic) is formed on amorphus silicon film 104 (FIG. 5C). Finally, amorphus silicon film 112 is then formed on dopant film 106 (FIG. 5D). Seam 116 having voids therein typically remains along the center line of the trench as a result of rough surface 114 (FIG. 4C). Heating the stacked layers of films 104, 106 and 112 tends to diffuse dopant film 106 into amorphus silicon films 104 and 112 to produce a conductive structure which conformably fills the deep trench and is useable as a capacitor electrode.
In this method of forming doped silicon films, the dopant film, often an arsenic layer, locally inhibits the growth of second amorphus silicon film 112. The second amorphus silicon film (i.e., amorphus silicon film 112) grows with discontinuous islands of growth due to the local inhibiting effects of dopant film 106 resulting in rough surface 114 (FIG. 4C). When such films are formed in deep trenches, seams and voids are formed along line 116 perpendicular to the semiconductor's principal surface (FIG. 5D).
The trench capacitor so formed has capacitor plate 98 and a capacitor electrode made of the conformally applied doped silicon film formed from films 104, 106 and 112 and has capacitor dielectric 102 disposed therebetween. This capacitor performs well; however, the seams and voids along line 116 may cause problems for subsequent semiconductor processing.
In FIG. 6A, substrate 100 is replaced with buried n-well 94 and p-well 96. Capacitor plate 98 is preferably formed of an n.sup.+ diffusion region formed in buried n-well 94. Other elements in FIG. 6A are the same as in FIG. 5B. In FIG. 6B, first doped silicon film 120 is formed from stacked films 104, 106 and 112 as described above with respect to FIG. 5D.
The subsequent processing steps include recess etching the trench, preferably by reactive ion etching (RIE). First doped silicon film 120 (FIG. 6B) is etched to form shortened first filling silicon 122 which is conductive just as film 120 is conductive. Because of the voids and seems in first doped silicon film 120, the recess etching process causes first rough notch 124 to be formed. In FIG. 6C, dielectric 102 is etched back as tar as is shortened first filling silicon 122. Alternatively, portions of dielectric 102 may be left on the entirety of the side walls of the deep trench.
Next, it is desirable to form oxide collar 126 (FIG. 6D) to insulated conductive first filling silicon 122 from p-well 96. The portion of the collar oxide which is largely parallel with the principal surface of the semiconductor is etched off. However, when forming collar oxide 126, first rough notch oxide 128 which is largely perpendicular to the principal surface of the semiconductor is unavoidably formed. If there were no first rough notch 124 (FIG. 6C) and first filling silicon 122 were to have a flat surface, it would be easier to avoid forming rough notch oxide 128 (FIG. 6D) since all of the surface of first filling silicon 122 would be largely parallel to the principal surface of the semiconductor. Thereafter, second doped silicon film 130 is formed from stacked films 104, 106 and 112 as described above with respect to FIG. 5D.
Again, recess etching is used to etch back second doped silicon film 130, preferably by reactive ion etching (RIE). Second doped silicon film 130 (FIG. 6D) is etched to form shortened second filling silicon 134 which is conductive just as film 130 is conductive. Because of the voids and seam in second doped silicon film 130, the recess etching process causes second rough notch 136 to be formed. It is desirable that first filling silicon 122 and second filling silicon 134 be electrically conductive; however, a reliable connection is difficult to achieve due to the formation of first rough notch oxide 128. If the seams and voids along line 116 (FIG. 5D) can be avoided, first rough notch oxide 128 can be avoided, and reliable electrical conductivity between first filling silicon 122 and second filling silicon 134 can be achieved. Furthermore, voids and seam in the second filling silicon can lead to cracks and dislocations in the device and a related leakage current.
In FIG. 6F, after second filling silicon 134 is formed, third filling silicon 138 is formed on second filling silicon 134, and third filling silicon 138 is recess etched, preferably by reactive ion etching (RIE). Third filling silicon 138 completes an electrical connection between second filling silicon 134 and source/drain region 140 of an MOS transistor.
In FIG. 6G, shallow trench 142 is formed to make a trench isolation barrier by forming oxide layer 144. Any voids or seams in second filling silicon 134 are also oxidized. The oxidation of seams and voids in second filling silicon 134 causes large stresses around the shallow trench. These stresses cause dislocations in the semiconductor material itself.